Ask a Question

NetworkAIR FM Secondary Compressor Lock Out

NetworkAIR FM Secondary Compressor will not Turn On

Issue:
If the suction pressure on an FM unit goes below 52 psi (on the newer firmware (6.15.0 ) the secondary compressor will turn off not come on.

What Happens:
If system suction pressure falls below 52 PSIG for more than three minutes, and the unit is running both compressors, the compressor currently deemed the 2nd stage is shut down. At the time of shut down, the control temperature reading is logged. If at any time after shutdown the control temperature rises two (2) degrees above the logged control temperature, the compressor currently deemed the 2nd stage is re-started. Short cycle start up delays apply.

Causes
• Lack of refrigerant: A system charge that falls below optimal will cause a low suction pressure condition. As the charge level continues to fall, suction pressures also continue to fall.
• Low return air temperatures: Under low return air temperature conditions, low evaporator load results in a new lower system balance temperature for the evaporator coil.
• Evaporator dirty: As airflow is reduced, total energy transferred to the air stream is also reduced. The result is again a low evaporator load that results in a new lower system balance temperature for the evaporator coil.
• Clogged filter drier: A partially clogged liquid line filter drier can cause a low suction pressure condition. As the clog continues to increase, refrigerant flow and hence suction pressure continues to decrease.
• Malfunctioning expansion valve: A sticking expansion valve sticking can cause a low suction pressure.
• Low condensing temperature: A low condensing temperature will cause a low suction pressure condition. The lower the condensing temperature, the lower the suction pressure.
• Short-circuited supply air: Supply air that is short circuiting back to the return air inlet has the same net result as low return air temperatures. The result is again a low evaporator load that results in a new lower system balance temperature for the evaporator coil.

Consequence
• Lack of refrigerant: System total capacity will be reduced. System sensible heat ratio will be reduced. Coil freeze up will occur if not addressed
• Low return air temperatures: System sensible capacity will be reduced. Coil freeze up will occur if not addressed. Compressor failure due to flood back* could occur if the evaporator coil becomes flooded.
• Evaporator dirty: System total capacity will be reduced. System sensible heat ratio will be reduced. Coil freeze up will occur if not addressed. Compressor oil return could be compromised due to reduced refrigerant flow. Compressor flood back* could occur if the evaporator coil becomes flooded.
• Clogged filter drier: System total capacity will be reduced. System sensible heat ratio will be reduced. Coil freeze up will occur if not addressed. Trigger of High head pressure switch cutout can occur if the level
of closure in the filter becomes severe.
• Malfunctioning expansion valve: System total capacity will be reduced. Under certain conditions, coil freeze up will occur if not addressed. Trigger of the high head pressure relief valve or fusible link in the receiver can
occur if the level of closure in the expansion valve becomes severe.
• Low condensing temperature: System total capacity can be reduced. System sensible heat ratio will be reduced. Coil freeze up can occur if not addressed. Compressor failure due to flood back* could occur if the
condensing temperature is dropped to extremely low conditions.
• Short-circuited supply air: System sensible capacity will be reduced. Coil freeze up will occur if not addressed. Compressor failure due to flood back* could occur if the evaporator coil becomes flooded.

NOTE: Environmental and system conditions that result in low system suction pressures can induce a coil freeze in HVAC systems that cause the system to shut down. The NetworkAIR FM has a unique method of system monitoring and control that prevents freeze up as a result of low suction pressure. This document outlines the causes of low suction pressure and the control strategies for prevention of coil freeze in the NetworkAIR FM.